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Subject: locking/rtmutex: Add adaptive spinwait mechanism
From: Steven Rostedt <rostedt@goodmis.org>
Date: Tue Jul 6 16:36:57 2021 +0200
From: Steven Rostedt <rostedt@goodmis.org>
Going to sleep when a spinlock or rwlock is contended can be quite
inefficient when the contention time is short and the lock owner is running
on a different CPU. The MCS mechanism is not applicable to rtmutex based
locks, so provide a simple adaptive spinwait mechanism for the RT specific
spin/rwlock implementations.
[ tglx: Provide a contemporary changelog ]
Originally-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/locking/rtmutex.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 49 insertions(+), 1 deletion(-)
---
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 59c36438428a..82f0a8209a41 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -8,6 +8,11 @@
* Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
* Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt
* Copyright (C) 2006 Esben Nielsen
+ * Adaptive Spinlocks:
+ * Copyright (C) 2008 Novell, Inc., Gregory Haskins, Sven Dietrich,
+ * and Peter Morreale,
+ * Adaptive Spinlocks simplification:
+ * Copyright (C) 2008 Red Hat, Inc., Steven Rostedt <srostedt@redhat.com>
*
* See Documentation/locking/rt-mutex-design.rst for details.
*/
@@ -1433,6 +1438,43 @@ static __always_inline int __rt_mutex_lock(struct rt_mutex *lock,
* Functions required for spin/rw_lock substitution on RT kernels
*/
+#ifdef CONFIG_SMP
+/*
+ * Note that owner is a speculative pointer and dereferencing relies
+ * on rcu_read_lock() and the check against the lock owner.
+ */
+static bool rtlock_adaptive_spinwait(struct rt_mutex *lock,
+ struct task_struct *owner)
+{
+ bool res = true;
+
+ rcu_read_lock();
+ for (;;) {
+ /* Owner changed. Trylock again */
+ if (owner != rt_mutex_owner(lock))
+ break;
+ /*
+ * Ensure that owner->on_cpu is dereferenced _after_
+ * checking the above to be valid.
+ */
+ barrier();
+ if (!owner->on_cpu) {
+ res = false;
+ break;
+ }
+ cpu_relax();
+ }
+ rcu_read_unlock();
+ return res;
+}
+#else
+static bool rtlock_adaptive_spinwait(struct rt_mutex *lock,
+ struct task_struct *owner)
+{
+ return false;
+}
+#endif
+
/**
* rtlock_slowlock_locked - Slow path lock acquisition for RT locks
* @lock: The underlying rt mutex
@@ -1440,6 +1482,7 @@ static __always_inline int __rt_mutex_lock(struct rt_mutex *lock,
static void __sched rtlock_slowlock_locked(struct rt_mutex *lock)
{
struct rt_mutex_waiter waiter;
+ struct task_struct *owner;
lockdep_assert_held(&lock->wait_lock);
@@ -1458,9 +1501,14 @@ static void __sched rtlock_slowlock_locked(struct rt_mutex *lock)
if (try_to_take_rt_mutex(lock, current, &waiter))
break;
+ if (&waiter == rt_mutex_top_waiter(lock))
+ owner = rt_mutex_owner(lock);
+ else
+ owner = NULL;
raw_spin_unlock_irq(&lock->wait_lock);
- schedule_rtlock();
+ if (!owner || !rtlock_adaptive_spinwait(lock, owner))
+ schedule_rtlock();
raw_spin_lock_irq(&lock->wait_lock);
set_current_state(TASK_RTLOCK_WAIT);
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